rpi: set board serial number in environment

Gets propagated into the device tree and then into /proc/cpuinfo where
users often expect it.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>